it's a gear with a ratio over 1:1 such as 0.90:1
your rear diff has a ratio
say.. 3.73 turns of the driveshaft to 1 turn of the axle
at the transmission, your gears start out low and go up, as a very basic example let's use
1st gear 4:1 2nd gear 3:1 3rd gear 2:1 4th gear 1:1 5th gear 0.90:1
in 4th gear if your engine is turning 3000 RPM so is your drive shaft and your effective ratio of rpms to your axle would be 3.73:1
make sense?
with automotive gear ratios a lower number is a "higher" gear , works very much like a 10 speed bicycle, in gear 1 your feet may be cycling extremely fast and you're barely moving, in 10th while difficult to pedal, your wheels end up turning much faster than your feet